This is a classic, similar to as suggested above, but with a bit more bite.
Watermelon Coolers
4 c peeled, seeded, cubed watermelon
1/2 c tequila*
1/4 c orange liqueur*
1T lime juice
1T sugar
Garnishes: lime wedges, watermelon, canteloupe, honeydew melon balls on skewers
*1/2c lemonade may be substituted for tequila and liqueur
Freeze cubs in ziploc bags or shallow pan for at least 6 hours. Position knife blade in food processor bowl. Add frozen watermelon cubes and the rest of the ingredients besides garnishes. Process until smooth, stopping to scrape down sides. Pour into glasses and garnish, if desired.